Anyway, we might have up to 60% calculating error... One person may have several wallets (and thousands of addresses inside 'em), there are lot of "technical" wallets for different services. It's all about crystal gazing.

We know that there are about 100,000 Mt. Gox accounts. If you assume (I'm just blindly guessing) that only every third Bitcoin owner also has an Mt. Gox account that would be 300,000 Bitcoin owners. Perhaps you could do a poll asking how many people do or do not have a Mt. Gox account, that would give you a better estimate.